One conventional method to avoid bridging loops is a spanning tree protocol (hereinafter, referred to as “STP”)
In common networks, bridging loops occur when redundancy is provided in consideration of device failures and the like. In view of this, with use of STP, the network system places one bridge port which is on the path with a loop connection into a blocking state, that is to say, logically cuts off a physically-connected path. This forms a tree-structured network topology in a logical sense, avoiding bridging loops as a result. Accordingly, there is only one logical path from a bridge to another bridge.
In addition, when a path in the network is cut off due to a failure or the like, the network system reconfigures the tree, that is, establishes a logical path by re-placing each bridge port into either a forwarding state or the blocking state.
In other words, use of STP enables a realization of networks which are free from bridging loops and ensure redundancy by means of path reconfiguration.
Also, in recent years, a bandwidth-guarantee type communication is emerging in the field of the network. This aims to provide, by guaranteeing a particular communication bandwidth, network environments suitable for multimedia data communication and the like which can perform continuous video transmission without interruptions.
However, when the bandwidth-guarantee type communication is to be performed, there is no guarantee for a required bandwidth to be allocated when necessary, as bandwidth resources of a transmission channel which interconnect bridges are shared by all the terminals.
Hence, for networks using STP, technologies have been developed to transmit data using a path other than the logical path established by STP, and are considered applicable to the bandwidth-guaranteed communication.
The first conventional technology aims to avoid delayed arrivals of packets by decreasing a load around the root bridge and the number of bridges for the packets to pass. A bridge that has a port thereof in a blocking state (hereinafter, referred to as “blocking port”) establishes a bypass path by referring to the routing information of the bridge to which the blocking port is connected (see Patent Document 1).
If a bandwidth can be allocated by using this bypass path, a bandwidth-allocated communication can be carried out by using the bypass path.
The second conventional technology aims to reestablish a bandwidth-allocated communication immediately after the network topology changes due to a path breakage or the like while the bandwidth-allocated communication is in progress. Specifically, this system allocates a bandwidth for paths including a blocking port in view of a possible path switch to these paths in the future (see Patent Document 2).
By using the communication path that is currently cut off by the blocking ports but has a transmission capacity preallocated thereon, the bandwidth-allocated communication becomes possible.
Note that the above-mentioned two technologies will be described later in detail using FIGS. 32 to 36.
Patent Document 1: Japanese Laid-Open Patent Application Publication No. H11-355337
Patent Document 2: Japanese Laid-Open Patent Application Publication No. 2005-102012